dm vdo: implement the volume index

The volume index is a large delta index that maps each record name to the
chapter which contains the newest record for that name. The volume index
can contain several million records and is stored entirely in memory while
the index is operating, accounting for the majority of the deduplication
index's memory budget.

The volume index is composed of two subindexes in order to handle sparse
hook names separately from regular names. If sparse indexing is not
enabled, the sparse hook portion of the volume index is not used or
instantiated.

Co-developed-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: J. corwin Coburn <corwin@hurlbutnet.net>
Co-developed-by: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Michael Sclafani <dm-devel@lists.linux.dev>
Co-developed-by: Thomas Jaskiewicz <tom@jaskiewicz.us>
Signed-off-by: Thomas Jaskiewicz <tom@jaskiewicz.us>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>